ASPHALTIC BITUMEN AS COLLOID SYSTEM’ J. PH. PFEIFFER A N D R. N. J. SAAL Laboratorium N . V . de Bataafsche Petroleum Maatschappij, Amsterdam, Holland Received August 7 , 198s
The conception due to Nellensteyn (2) that asphaltic bitumens must be looked upon as colloid systems has been further developed by Pfeiffer and his collaborators ( 5 ) in the Laboratory of the N. V. de Bataafsche Petroleum Maatschappij a t Amsterdam (the Netherlands). According to these theories, the great differences in rheological properties found can be accounted for by the fact that asphaltic bitumens consist of dispersions of micelles in heavy, purely viscous oils and that these micelles are peptized to different degrees resulting in differences in the tendency to form gels. To make the discussion in the subsequent pages quite clear, it is necessary first to give details about our conception of such systems, after which a brief survey will be given of experimental data which fully substantiate these ideas. A more extensive survey will shortly appear elsewhere. I. GENERAL CONCEPTION O F THE CONSTITUTION O F ASPHALTIC BITUMEN8
The majority of asphaltic bitumens contain substances which are insoluble in low-molecular aliphatic hydrocarbons, and which when heated do not soften, but decompose, swell, and finally sinter together. This fraction is generally called the “asphaltene” fraction, as distinguished from the soluble or “maltene” fraction consisting of heavy, purely viscous oil. The asphaltenes in all probability consist of high-molecular hydrocarbons of a predominantly aromatic character, with a comparatively low hydrogen content, formed by condensation and dehydrogenation from aromatic-naphthenic hydrocarbons of lower molecular weight. As a rule some oxygen, some sulfur, and a still smaller quantity of nitrogen are present. Depending upon the intensity of the heat treatment to which the base material or the bitumen formed from it has been subjected, different asphaltenes are obtained which comprise a continuous range of products of increasing carbon content up to “carbenes,” “carboids,” and ultimately coke. The latter are especially produced by high level cracking according Presented a t the Sixteenth Colloid Symposium, held a t Stanford University, California, July 6-8, 1939. 139
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J. PH. PFEIFFER AND R. N. J. S M L
to the Dubbs process. Even this so-called coke is not pure elementary carbon, but still contains a certain proportion of hydrocarbon chains or rings, which may be quite firmly bound to a carbon nucleus (4). In the discussion which follows, it is quite immaterial whether the binding of these hydrocarbons is of a predominantly chemical or physical nature. A point of great importance is that most asphaltenes have a marked tendency to adsorb aromatic hydrocarbons of lower molecular weight,
FIQ.1. Schematic representation of peptized asphaltene micelles and that this faculty is the more pronounced the less severe the heat treatment has been. The asphaltenes, as they are found in natural crudes or in carefully steam-distilled residues, represent the first stages of condensation and have dimensions that defy observation under any ordinary or dark-field microscope. In the asphaltic bitumens the asphaltenes are the centers of micelles which are formed by adsorption, and perhaps partly by absorption, of part of the maltenes on the surfaces or in the interiors of the asphaltene
ASPHALTIC BITUMEN AS COLLOID SYSTEM
141
particles. This part of the maltenes consists chiefly of hydrocarbons of an aromatic or of a combined aromatic-naphthenic-aliphatic nature and is by some authors referred to ais resins. The other part of the maltenes is often referred to as oily constituents or petrolenes. According to our conception, the structure of such a micelle is such that the bulk of the substances with the greatest molecular weight and with the most pronounced aromatic nature are arranged the most closely to the nucleus. These again are surrounded by lighter constituents of less
FIQ.2. Schematic representation of flocculated asphsltene micelles aromatic nature and so on, until a gradual and nearly continuous transition to the intermicellar phase is formed (figure 1). In other words, there is no distinct interface; neither a t the outside of the micelle, nor around the nucleus, is there a contact of Substances with distinct differences, for instance, in surface tension. It is therefore clear that it is impossible to make a sharp and rigid distinction between fractions such as the socalled resins and the oily constituents. When the entire system contains sufficient constituents for the formation of the outer regions of the micelles, the asphaltenes are fully peptized and able to move through the bitumen as freely as the viscosity of the
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intermicellar phase permits. Such a system will show almost purely viscous flow, except for a marked intramicellar elasticity (3). If, however, there is a shortage of “asphaltic resins,,’ part of the forces causing the formation of the micelle are not compensated by adsorption of asphaltic resins, and the micelles will be subjected to mutual attraction. The micelles will form a kind of bond (figure 2) in places where they accidentally approach so closely as to get into a region of minimum potential energy formed by the counterbalance of mutual attractive and
FIG. 3. Schematic representation of the structure of an asphaltic bitumen of the blown type
repulsive forces. A certain force is then required to remove them from this region. Thus a gel structure is formed (figure 3) that may be described as irregular open packing, the spaces of which are filled by the intermicellar liquid. Such a system may exhibit all the characteristics of complex flow, such as elasticity and thixotropy. In this case the elasticity is composed of intramicellar and intermicellar elasticity, There is reason to presume that the intramicellar elasticity predominates and that its effect is enhanced by the presence of the structure and its (intermicellar) elasticity. A more detailed discussion of such systems and their properties is given by one of us elsewhere (3).
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I n the following pages a brief survey will be given of the experimental data on the properties of asphaltic bitumens which have been obtained by the authors and their collaborators. 11. PROPERTIES OF ASPHALTIC BITUMENS AND OF THEIR COMPONENTS
The usual way to discriminate between different asphaltic bitumens is according to hardness, which is in practice expressed by the penetration of a standard needle under standard conditions. Bitumens of the same hardness frequently show differences in other rheological properties. In a previous publication by Pfeiffer and van Doormaal ( 5 ) it has been explained that for practical purposes the temperature susceptibility of asphaltic bitumens can be expressed by a single number, the penetration index (P.Z.). I n the form of an index figure it gives the mean slope of the log penetration-temperature curve in the temperature interval between 15" or 25°C. and the temperature of the softening point R & B. TABLE 1 Relationship between properties NATURE OF THE MATERIAL
ie TEAT OF
ATUBE ~~
-2.5 0
0.060 0.040
2
0.030 0.020
5
None Moderate Rather great Great
None Slight iModerate Great
Coal-tar pitch Normal asphaltic bitumen Rubber
A drop of one point in P.I. approximately corresponds to a rise of 15 per cent in (d log penetration)/(d temperature). As has been stated in the introduction, deviations from purely viscous flow are assumed to be due to the colloid nature of the bitumen, and to be particularly great when the micelles form more or less coherent structures. Thus the temperature-susceptibility as indicated by (d log penetration)/ (d temperature) and P.Z. is related t o the degree of elasticity and thixotropy attributable to the micellar structure of the mass. These relationships are shown in table 1. The best way of splitting asphaltic bitumen into fractions is by mixing them with an excem of some low-boiling saturated hydrocarbon. The quantities of the soluble part (the maltenes) and the insoluble part (the asphaltenes) vary for the same bitumen with the nature and the quantity of solvent used, with the temperature of mixing, and with the time elapsing between mixing and separation of the precipitate. Therefore the procedure has t o be thoroughly standardized.
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We shall refer to these fractions as 60/80 asphaltenes or pentane asphaltenes, according as an aromatic-free gasoline having a boiling range of 60’ to 80’C. or pentane is used as a solvent. We have studied such properties of these fractions as are expected to influence the properties of the whole system, by examining fractions obtained from several bitumens produced by different processes and with widely varying rheological properties.
Asphaltenes The important properties of the asphaltenes are the chemical composition and the particle size and shape. Little difference was found between the C/H ratios of the asphaltenes of most steam-refined and blown products examined; these varied in most cases from 0.8 t o 0.9. The asphaltenes from cracked residue showed far higher ratios up to about 1.3. Different fractions obtained from single asphaltenes by extraction usually show only slight differences in C/H ratios. The influence of the C/H ratio on the ability of the asphaltenes to adsorb aromatic compounds was investigated by studying the relative viscosities of dispersions of different asphaltenes in benzene. Evidence was found that micelles formed with asphaltenes having a C/H ratio of less than 0.9 occupy about four times the volume of the asphaltenes alone as originally added, and that the asphaltenes having a C/H ratio of over 1.1 occupy only about twice the volume of the original asphaltenes. These conclusions depend upon the assumption, for the validity of which there are several indications, that the shape of the bitumen micelle approaches that of a sphere. Now and again, widely different data on the “molecular weight” of asphaltenes have been published. From our point of view it is more correct to speak of the “mean micelle weight.” It will, however, be clear that it is impossible to conceive of the “micelle” as a sharply delimited unit. Not only do the micelles perhaps differ widely in size, but their composition depends on the medium in which the asphaltenes are dispersed. By solving asphaltenes in some solvent, part of the high-molecular protective substances (asphaltic resins) will, dependent on the adsorption equilibrium, remain attmhed to the asphaltene nuclei and form part of the micelles, whereas the remainder will be present in a molecularly dispersed condition in the liquid phase. It will be clear from the above that the usual methods of determining molecular weights by measuring the depression of freezing points or the lowering of vapor pressures will at most give only rough indications of the micelle size. Figures ranging from about loa t o l(r are usually obtained b y these methods. An attempt to estimate the micelle size of asphaltenes by studying the
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ASPHALTIC BITUMEN AS COLLOID SYSTEM
behavior of “monomolecular” films according to the method of Langmuir (1) gave the values collected in table 2. The results found by this method were not influenced by variations in the concentration of the solution, in the area over which the solution was spread, or in the duration of the test. It is, however, clear that these figures can give only a rough estimation of the mean weight of the micelle. The values found are probably too high, as deviations from spherical shape (though too small to influence viscosity) as well as the packed arrangements of large and small particles in the film may greatly affect the results. Maltenes From our conception of the structure of asphaltic bitumens it follows that the peptizing power of the maltenes depends mainly on (1) the TABLE 2 Size of micelles of asphaltemr MPEALTENES
Mexican, steam-refined. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...................................... Venezuelan, steam-refined.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Venezuelan, blown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Blown East India distillate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Egyptian, cracked. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
80,000
1 1 ~
110,000 140,000 9,000 18,000
content of aromatics, by which is meant the proportion of the maltenes consisting of aromatic rings, and ( 2 ) the molecular weight of the aromatic fractions. To verify these ideas a number of asphaltic bitumens of different types and with divergent rheological properties were split into a number of fractions rich and poor in aromatics by means of solvent extractions. The aromaticity of these fractions was further judged by various methods. The study of these data is too complicated to be discussed here. Bitumens of the gel type (generally blown bitumens) have been found to contain only small amounts of high-molecular liquid aromatic compounds (or resins) and to contain maltenes which are comparatively poor in aromatics, whereas all of the constituents of bitumens derived from cracked residues, which bitumens are of the sol or coal-tar pitch type, are found to be of a predominantly aromatic nature. 111. RELATIONSHIP BETWEEN
THE PROPERTIES O F ASPHALTIC BITUMENS
AND THEIR COMPOSITION
In order to establish the influence of the nature of the components on the properties of asphaltic bitumens, we have investigated “synthetic”
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products made by combining maltene fractions and asphaltene fractions, which fractions were obtained by solvent extractions of representative asphaltic bitumens. These fractions were derived in some cases from a single bitumen and in other cases from different bitumens. The preparation of synthetic asphaltic bitumens was effected by dissolving all of the components together in carbon disulfide and then distilling off the carbon disulfide. Blank experiments in which bitumens were split into various fractions and subsequently recomposed have shown slight increases in hardness, but never changes of the characteristic rheological propertiex which together determine the type of a bitumen. By omitting certain fractions and comparing the properties of the products thus obtained with the “complete product,” we were able to determine the influence of these omitted fractions. By replacing such fractions by similar ones from other types of bitumens it was possible to compare their influences on the resulting systems. The rheological properties of the synthetic bitumens thus obtained were examined by means of the conicylindrical rotation viscometer (6). With the data thus obtained it could be shown that the nature of the maltenes has a marked influence on the tendency of asphaltic bitumens to form gel structures. For a given asphaltene content this tendency is absent in the presence of a certain excess of aromatics and increases as the aromatic content of the maltenes is lowered. It has further been demonstrated that the particular nature of the asphaltenes is of importance. Asphaltenes having a relatively low C/H ratio are easily solvated and hold the solvation mantles firmly, but when the maltenes contain insufficient lower aromatic constituents to supply a mantle, these asphaltenes lead to the formation of gels. Asphaltenes having a high C/H ratio require maltenes of high aromatic content to form stable sols, and with maltenes of low aromatic content flocculation rather than gelation may occur. The latter fact is the basis of the “Oliensis” test, which aims a t showing the presence of cracked material in an asphaltic bitumen. The high elastic deformability of bitumens may be ascribed to intramicellar and intermicellar elasticity. Intramicellar elasticity, as measured by elastic recovery, increases regularly with the shearing stress. Intermicellar elasticity, although more marked at low shearing stresses, is limited by structural failure. The intramicellar elasticity decreases with increasing C/H ratio of the asphaltenes; this may be attributed to smaller micelles, or to more compact structure of the micelles, or to combinations of these. IV. EQUILIBRIA I N THE MALTENE-ASPHALTENE SYSTEM
The relative volumes of the two phases in systems composed of asphaltenes and maltenes is of importance, but it is difficult to determine them
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ASPHALTIC BITUMEN .4S COLLOID SYSTEM
because they are always altered by any change to which the system is subjected. The quantities of precipitate obtained by mixing with other liquids such as aliphatic hydrocarbons give little information as to the total volume of the micelles. These ratios can be found, however, at least for bitumens of the blown type (with a distinct gel structure), by taking advantage of the phenomenon of "sweating", which is similar to the syneresis of aqueous gels. Sweating involves the separation of small quantities of the lntermicellar phase from thc system, and is duc presumably to contractive forces in the micelle skeleton. Seither the usual asphaltic bitumens nor those examined here exhibit this phenomenon to the extent that they yield separated intermicellar liquid of themselves. I t is, however, possible to effect this separation by contact with porous materials. This was done by spreading a suitable powder evenly on a layer of the bitumen being examined, and, after contact for a given time a t a specified temperature, removing the powder and carefully extracting from it the sweated oil. TABLE 3 Comparison of maltenes with ozl sweated at 60°C.
.I
Density, 25"/4". , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , . , , , Percentage by weight of saturated hydrocarbons present Per cent by weight insoluble in W/SO gasoline. . . . . . . . . , .
~
yn"lT/,*,, 0.963 39 0.0
I ~
SWEATED OIL
0.943 490.2
The properties of the sweated oil thus obtained were found in most cases to differ materially from those of the maltenes separated from the bitumen with 60/80 gasoline, which shows that the composition of the micelles differs from that of the 60/80 asphaltenes. For a blown Mexican bitumen containing 32 per cent by weight of 60/80 asphaltenes, figures comparing the maltenes with the oil sweated at 50°C. are given in table 3. The sweated oil proves to be practically free from asphaltenes, showing that the continuous phase of the system has therefore actually been separated off. From these figures the quantity of intermicellar liquid can be calculated, if it be assumed that no saturated hydrocarbons have been bound by the micelles. By this method we have examined various bitumens of the same P.Z., hence of nearly the same rheological properties and the same type, but differing in asphaltene content. The data obtained are given in table 4 and point to substantial quantitative differences in the ratio of the phases. Thus, bitumen No. 1 has a much smaller quantity of intermicellar liquid than No. 3. In agreement with this, No. 1 shows a lower rate of oil
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J. PH. PFEIFFER AND H. N. J. BAAL
sweating, although the viscosity of the sweated oil is lower than that of No. 3. Moreover, the specific gravity of the sweated oil from No. 3 is lower than that of No. 1, which points to a more saturated character of the sweated oil from No. 3. This accounts for the fact that these bitumens do not differ much in rheological properties, as is roughly indicated by the P.Z. The more saturated nature of the intermicellar liquid implies that the mutual binding in the structure is stronger and thus the smaller quantity of micelles in No. 3 are yet able to form a structure with about TABLE 4 Comparison of the quantities of intermicellar liquid i n bitumens of the same t y p e and hardness '
BITUMEN NO.
ANALYsll
Density, 25"/4".. . . . . . . . . . . . . . . . . . . . . . . . . 1.044 R & B melting point, " C . .. . . . . . . . . . . . . . . 86.5 Penetration at 25"C.,. . . . . . . . . . . . . . . . . . . 24 P . I . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Viscosity, 150"C.,centistokes. . . . . . . . . . . . . 5400 Per cent by weight insoluble in 60/80 32.2 gasoline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SWEATIN0 TESTS
1.028 85.5 25.5 3.4 3900
1.019 86 29 3.8 2530
30.4
26.7
0.45 0.943 310
0.88 0.938 910
I
Quantity of sweated oil, 10 days at 50°C... . Density, 25'/4". . . . . . . . . . . . . . . . . . . . . . . . . . . . Viscosity, 50"C., centipoises. . . . . . . . . . . . . . . Quantity of intermicellar liquid: Per cent by weight at 50°C... . . . . . . . . . . . Per cent by volume at 50°C.. . . . . . . . . . . . .
0.25 0.953 230 36 39.5
52 56
About 58 About 63
the same rheological properties as No. 1. In addition, the particular nature of the micelles may be a factor, the asphaltenes of No. 3 having absorbed fewer constituents of the maltenes, and being thus less "protected." This would also be expected for asphaltenes differing in composition, It follows from these few examples that a t normal temperature the micelles in asphaltic bitumens consist of asphaltenes to which a substantial portion of the maltenes may be bound. Since the quantitative ratio between the volume of the micelles and asphaltenes is not constant, the quantity of asphaltenes is to be considered only as a very approximate indication of the micelle content.
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REFERENCES (1) I.: J. Am. Chem. SOC.39. 1848 (1917). . . LANGMUIR. (2) NELLENSTEYN, F. J. : Bereiding en constitutie'van asphalt (Manufacture and constitution of asphaltic bitumen); Dissertatie Technische Hoogeschool, Delft, 1923. (3) PFEIFFER, J. PH.: De Ingenieur 64, P4-P10 (1939). (4) PFEIFFER, J. PH.: De Ingenieur 64, Mk41-Mk47 (1939). ( 5 ) PEFIFFER, J. PH.,AND DOORMAAL, P. M.VAN:J. Inst. Petroleum Tech. 22, 414 (1936). 16) SAAL,R. X , J., AND LABOUT, J. W. A.: J Phys. Chem. 43, 149 (1939).
RHEOLOGICAL PROPERTIES OF ASPHALTIC BITUMENS' R. N. J. SAAL
AND
J. W. A. LABOUT
Laboratoraum N . V . de Bataafsche Petroleum Maatschappij, Amsterdam, Holland Received August 7, 1959 I. INTRODUCTION
In numerous investigations, the rheological properties of asphaltic bitumens have been found to vary widely in elastic deformability and thixotropy (5, 6). From the point of view of their thixotropic properties, they may be divided arbitrarily into two groups-those of the sol type and those of the gel type (3). Elasticity may be due to the elastic deformability either of the separate micelles or of a structure built up by coherent micelles; it may therefore occur both in bitumens of the sol type and in those of the gel type (4). The wide and continuous range of chemical composition observed in all asphaltic bitumens makes it likely that free (sol) micelles or isolated small agglomerates of micelles, which may themselves be considered free micelles, may be present in bitumens of the gel type as well as in those of the sol type. Among the technical asphaltic bitumens, therefore, many representatives of mixed gel-sol types may be expected. In order to be able to judge in how far the rheological properties of asphaltic bitumens may be explained by regarding them either as sols or as gels, a close study was made of the rheological properties of two quite different bitumens, not only the deformation under constant shearing stress being investigated, but also the elastic recovery or the relaxation of the stress. The measurements were carried out in a concentric rotation viscometer with a conical bottom. Presented a t the Sixteenth Colloid Symposium, held a t Stanford University, California, July 6-8, 1939.
